1. Field of the Invention
This invention relates to a technology and a circuit board for reducing the impedance of a power supplying system of the circuit board as well as an electronic apparatus adopting the technology and employing the circuit board. More particularly, the present invention relates to a technology suitable for reducing the impedance of a power supplying system and the amount of radiated noise (emission noise).
2. Description of the Prior Art
An electronic circuit operates by supplying a voltage and a current from a power supply. In the following description wiring connecting the power supply and the electronic circuit is referred to as a power supplying system. Let the voltage supplied by the power supply be denoted by V an operating current of the electronic circuit be denoted by ic, the internal impedance of the power supply be denoted by Zp and the impedance of the power supplying system other than the internal impedance Zp be denoted by Z1 as shown in FIG. 8. In this case a voltage Vc actually applied to the electronic circuit is expressed by Eq. (1) as follows:
Vc=Vxe2x88x92ic((Zp+Z1)xe2x80x83xe2x80x83(1)
If the product of the operating current ic and the sum of the impedances Zp and Z1 is large, the actually applied voltage Vc drops, causing the electronic circuit not to operate normally. The product of the operating current ic and the sum of the impedances Zp and Z1 is referred to as a power-supply voltage drop. For a fixed operating current ic, it is necessary to reduce the sum of the impedances Zp and Z1 shown in Eq. (1) in order to make the power-supply voltage drop small. In order to reduce the sum of the impedances Zp and Z1, it is necessary to decrease the impedance Zp of the power-supply circuit and the impedance Z1 of the wiring of the power supplying system.
There are three implementable methods of reducing the impedance of the power supplying system. According to the first method, the area of the cross section of the wire of the power supplying system is increased. In the second method, the power supply is placed at a location close to the ground. According to the third method, the length of the entire power supplying system, that is, the wiring, is reduced. As a combination of the three methods there is known a conventional method wherein a, bypass capacitor is implemented on a multilayer circuit board at a location in close proximity to the electronic circuit as shown in FIG. 9. The multilayer circuit board includes a plane power-supply layer and a plane ground layer. Since the power-supply and round layers are made plane the areas of the cross sections are large and the power-supply and ground layers can be placed at locations close to each other in a parallel orientation. As a result the inductance can be reduced. In addition by implementing the bypass capacitor at a location in close proximity to the electronic circuit the bypass capacitor plays a substitutive role of the power supply that corresponds to a high-speed power-supply current and thereby the power supply becomes a short distance from the electronic circuit. Thus these effects allow the impedance of the power supplying system to be reduced.
By using a multilayer circuit board the impedance of the power supplying system can be reduced as described above. In addition, by interfacing the power-supply layer with the ground layer over a wide area the power supply system (the multilayer circuit board) exhibits a behavior of a distributed constant circuit in a high-frequency zone. Thus, noise generated at a point on the power-supply layer propagates through the power-supply and ground layers, being reflected at the ends of the board. The reflection of the noise at the ends of the board is repeated, resulting in a standing wave on the power-supply and around layers. In turn, this standing wave raises a technical problem of a seemingly increased impedance of the power supplying system. This appearance of the standing wave is referred to as a parallel resonance, phenomenon or an antiresonance phenomenon. FIG. 10 is a diagram showing the state of the standing wave. With a standing wave generated as described above, the power-supply and ground layer are equivalent to a noise source 907a connected to a noise source 907b in series as shown in FIG. 11.
Methods for solving this technical problem include a technique of providing a termination at each edge of the board as is disclosed in Japanese Patent Laid-Open No. Hei 8-64984 and a technique of forming the board into a hexagonal shape as is disclosed in Japanese Patent Laid-Open No. Hei 9-246681 to make resonance difficult to occur. With the former method disclosed in Japanese Patent Laid-Open No. Hei 8-64984 however, the layer structure of the board becomes complicated, raising a technical problem of an increased price of the board. As for the latter method disclosed in Japanese Patent Laid-Open No. Hei 9-246681, there is raised a technical problem of lower wiring and implementation efficiencies.
In addition, the resonance of the power-supply and ground layers causes the following technical problem. If a signal cable is connected to the circuit board as shown in FIG. 12 the ground can be represented by an equivalent circuit shown in FIG. 13. As shown in the figure the series circuit comprising the noise source 907a and the noise source 907b is connected to a ground line 906 of the cable. The noise source 907a and the noise source 907b excite the ground line 906 to work as a dipole antenna, generating an EMI a kind of undesired electromagnetic radiation. In particular if an antiresonance phenomenon occurs a standing wave generated on the ground layer serves as these noise sources raising a concern about generation of a large EMI.